In an electric motor driving system in which an electric motor is driven at a variable speed by a power converter to operate a rotating apparatus such as a fan, a pump and a compressor through a reduction gear, an acceleration gear and the like, an oscillating component having a specific frequency can appear at a shaft between the electric motor and the rotating apparatus. Such a frequency for oscillation is unique to the apparatus, and power converter driving needs to be controlled so that an oscillation having such an unique frequency does not adversary affect the system.
For this reason, there is a known art for controlling the output frequency of a power converter to shorten (skip) an operating time at a specific frequency in which the oscillation is large. There is also a known art for controlling a power converter to cancel the above-described oscillating component in driving in the neighborhood of a specific frequency, in which the speed fluctuation or torque fluctuation component is estimated so that a reverse torque by the electric motor is applied to cancel the pulsation, as is disclosed for example in Patent Literature 1.
Also, in Patent Literature 2, oscillating components are suppressed by controlling voltage for driving an electric motor so that the phase of an oscillating component included in motor torque or motor torque current generated by the electric motor advances by 90 degrees or more with respect to the phase of an oscillating component of identical frequency included in a rotating electric angular frequency of the electric motor.